How do the analgesic effects of cannabis relate to the brain’s neural activity?

Weizman L, Dayan L, Brill S, Nahman-Averbuch H, Hendler T, Jacob G, Sharon H. Cannabis analgesia in chronic neuropathic pain is associated with altered brain connectivity. Neurology 2019;91:e1285-e1294 (abstract)

Why was this study conducted?

There is a lot of interest in the ability of cannabis, particularly its main psychoactive substance Δ-9-tetrahydrocannabinol (THC), to relieve pain.  In this randomized, double-blind, placebo-controlled cross-over trial, researchers conducted functional magnetic resonance imaging (fMRI) of the brain before and two hours after sublingual administration of THC or a placebo.  Subjects were 15 men aged 27-40 years with chronic radicular neuropathic pain (i.e., pain due to inflammation or other irritation of a nerve root at its connection to the spinal cord).

What does this study add?

The study found patients’ subjective reporting of pain correlated with a reduction in connectivity between two parts of the brain associated with pain processing: the anterior cingulate cortex (ACC), which is involved in processing the affective aspects of pain, and the sensorimotor cortex, which plays a role in the sensory aspects of pain.  The authors suggest pain relief may occur because THC disrupts synchrony and integration between these two pain-processing pathways. In addition, THC may reduce activity in the dorsolateral prefrontal cortex (DLPFC). These findings suggest a neurologic explanation for the analgesic effects of THC.

Is there anything else I should know?

This was a small study (n=15) that only included men with a specific type of neuropathic pain. More research will be needed to validate these findings and determine whether they are applicable to women and other forms of pain.